1. Field of the Invention
This invention relates to a method and apparatus for grinding the teeth of a circular saw blade to an elliptical contour.
2. Description of the Prior Art
Circular saw blades and methods and apparatus for finishing the teeth thereof are quite common. Circular saw teeth with carbide inserts, initially having a cylindrical or spherical shape are known to those of ordinary skill in the art. For example, Goehle, U.S. Pat. No. 2,675,603 discloses the use of a cylindrical carbide element as a tooth in the context of a composite circular saw. While the insert is cylindrical in shape, it is eventually ground and faced so as to have a conventional working surface. Kolesch, U.S. Pat. No. 3,537,491 and Tyler, U.S. Pat. No. 3,788,182 both disclose the use of spherical carbide ball inserts adapted for use in other circular saws. However, as in the case of Goehle, the spheres are subsequently faced and the remaining rounded section of the sphere is not in actuality the working surface. Lewis, U.S. Pat. No. 485,503, Bruestle et al., U.S. Pat. No. 2,860,863 and LaVelle, U.S. Pat. No. 3,380,497 all disclose prior art circular saws having a rounded profile when seen from either the front or the back of the tooth. However, the working surface is not a continuous curve. Osgood, U.S. Pat. No. 238,521 and Pioche, U.S. Pat. No. 1,334,633 likewise disclose saws in which the side view profile of the tooth is rounded or cylindrical in some aspects but wherein the working surface is not a continuous curve.
The problem encountered with old-fashioned carbide tooth saws in two-fold. One of the problems is that the carbide insert is hard and brittle and cannot take interrupted cutting. During interrupted cutting the shock loading on the teeth is so great that the teeth will frequently break. The second problem associated with conventional carbide insert saws is that carbide has an affinity for many metals that it cuts. Frequently, a metal chip will get stuck on the face of the carbide insert or in the gullet thereunder. According to the prior art methods of cutting, it was often necessary to get rid of this excess metal by means of an air blast or brushes. Unfortunately, these techniques were not always satisfactory and resulted in great blade deterioration.
Recently, a saw blade having an improved metal cutting geometry was disclosed in U.S. Pat. Nos. 3,576,061 and 3,619,880 to Pahlitzsch. This type of saw blade is often referred to as the "German" geometry. According to those inventions, an improved cutting characteristics could be achieved by slightly beveling the top face of the tooth down toward the side of the tooth. This technique is similar to the technique of "breaking the corners" in order to prolong saw blade life. While the saws disclosed in U.S. Pat. Nos. 3,576,061 and 3,619,880 do display some improved cutting characteristics, they are nevertheless difficult to manufacture because they require several different grinding steps in order to achieve a satisfactory geometry. What was desired in this context was a saw blade that exhibited cutting characteristics better than those disclosed in U.S. Pat. Nos. 3,576,061 and 3,619,880 and which were easier to grind and regrind. The following patents are also of note in that they describe relevant structures: Aber, U.S. Pat. No. 2,344,954; Horth, U.S. Pat. No. 2,381,540; Soderstrom, U.S. Pat. No. 3,878,747; Haycock, U.S. Pat. No. 2,546,503; Kaiser, U.S. Pat. No. 3,434,190; Maurer, U.S. Pat. No. 2,831,240 and Shephard, U.S. Pat. No. 2,891,300.
A saw blade having characteristics better than those ascribed to the German Geometry was subsequently invented by Robert H. Nowak. That blade is described in pending application Ser. No. 587,504 filed June 16, 1975. Nowak discovered that an improved working surface can be formed by grinding a portion of the face of the tooth to a rounded profile. It was found that saw blades produced with teeth having a working surface with a continuously curved cross-section exhibited superior steel cutting characteristics and were relatively easy to grind and regrind. However, it was found that the machinery necessary to produce that type of saw blade has certain undesirable limitations. Accordingly, it is necessary to describe the limitations of the prior art saw grinding equipment in order to appreciate the full value of the present invention.
The practice of grinding circular saws with abrasive wheels is known to those of ordinary skill in the art. Grinding equipment is made, for example, by the Vollmer Corporation of Dornhan, West Germany, and others. One of the major problems associated with conventional prior art carbide tipped steel cutting circular saws in that they cut slowly and wear out quickly. This difficulty was greatly reduced by the discovery that if the top of the face of the saw teeth are specially shaped to have a more negative rake angle relative to the rake of the rest of the tooth face, then efficiency was greatly improved. The special shaping typically involved multiple grinding operations in which several flat facets were formed in the top of the carbide tip. While the new shape was superior to the old shape, it was nevertheless time consuming to grind due to its multi-faceted contour. A description of this prior art shape may be found in an article by Pahlitzsch and Willemeit which appeared in Volume 58, Issue 4 of Werkstattstechneik (1968).
The Nowak saw blade was discovered to be superior in many characteristics to the saw blade generated by the "German" Geometry and disclosed in the patents to Pahlitzsch, U.S. Pat. Nos. 3,576,061 and 3,619,880. A device to economically and efficiently produce the Nowak saw blades was thereafter invented by Horst Von Arnauld and Daniel J. Murphy. A description of the Von Arnauld and Murphy invention may be found in copending U.S. application Ser. No. 540,449 filed Jan. 13, 1975 and entitled "Method and Apparatus for Grinding the Teeth of a Circular Saw to an Improved Contour", the entire disclosure of which is incorporated herein by reference. The Von Arnauld and Murphy invention is directed toward the automatic machinery which grinds the circular saw teeth to the contour described in the Nowak invention. The automatic apparatus and method comprehends a cup-shaped grinding wheel having an abrasive inner rim which is automatically brought into grinding contact with a tooth of a circular saw. A working surface having a continuously curved contour is thereby generated. The working surface is further characterized in that it typically has a more negative average rake angle than the remaining portion of the face of the tooth. The invention further comprehends the use of a programmed electronic unit which automatically controls the relative movements of the grinding wheel and the saw blade during the grinding operation. According to the Von Arnauld/Murphy invention a broad section of the abrasive inner rim of the cup-shaped grinding wheel comes into substantially simultaneous contact with the face of the saw tooth. The grinding wheel therefore plunges into the face of the tooth and remains there for as long as necessary to effectuate the necessary grinding. After the tooth is ground the grinding wheel is subsequently retracted and the new tooth is indexed into grinding position. One major difficulty experienced with the Von Arnauld/Murphy invention is that the grinding wheels were subjected to a great deal of wear. Since the same part of the grinding wheel always came into contact with the face of the saw teeth, the grinding wheel itself eventually took on the characteristic contour of the tooth to be ground. Accordingly, subsequent teeth ground with the same grinding wheel often assumed the characteristics of the grinding wheel itself and as the grinding wheel became progressively worn the contour imparted to the teeth progressively deteriorated. In addition, while the automatic method and apparatus invented by Von Arnauld and Murphy made it possible to grind teeth more efficiently, there were still some time limitations which decreased the overall speed of the machinery. Accordingly, new methods were sought to improve upon the previous method and apparatus.
The present invention is also a partial outgrowth of experiments made by Mr. Robert Nowak to manually edge grind saw blades to the improved rounded geometry.
In addition to the foregoing prior art the following patents appear to disclose relevant structure and methodology: Armstrong, U.S. Pat. No. 1,771,602; Markhe, U.S. Pat. No. 2,874,517; Segal, U.S. Pat. No. 2,958,240; English, U.S. Pat. No. 3,304,810; Drake et al., U.S. Pat. No. 3,313,185; Daggett, U.S. Pat. No. 3,616,711 and Brenner, U.S. Pat. No. 3,766,806.
It was in the context of the foregoing prior art that the present invention arose.